1. Field of the Invention
The present invention relates to a healing agent in cement-based materials and structures, as well as to a process for its preparation.
2. Description of Related Art
Porous aggregate material (expanded clay- or sintered fly-ash) loaded with bio-chemical compounds (bacteria and/or organic compounds) can improve the durability of cement-based structures when incorporated in the material matrix. Porous materials such as different types of expanded clays (brand name, e.g., Liapor®, Argex®) and fly-ash (sintered pulverized coal ash) (e.g., Lytag®) are commonly applied as aggregate material in cement-based materials, specifically for the production of lightweight concrete. So far, however, the potential storage capacity of these porous materials for healing or repair agents, such as chemical compounds or bacteria, have not been proposed or applied yet.
In recent years, the application of bacteria for the improvement and/or repair of cement-based materials, and concrete in particular, have been investigated in several studies (Bang et al. 2001; Ramachandran et al. 2001; DeMuynck et al. 2005 and 2007; Jonkers & Schlangen 2007a+b; Jonkers 2007). In some of these studies bacteria, or derived enzymes, were applied externally, i.e., as a surface treatment system, to plug, seal, or heal cracks in concrete through metabolic or enzymatic biomineral formation. In only few reported studies bacteria were truly incorporated in the concrete matrix (e.g. by mixing with the still fluid cement paste), to investigate their potential for autonomous improvement of concrete characteristics, e.g. to act as concrete-immobilized self-healing agent (Jonkers & Schlangen 2007a+b; Jonkers 2007).
Major disadvantage of direct addition of bacteria or their spores to cement paste is that this procedure may strongly decrease their viability [Jonkers & Schlangen 2007b]. Reason for the limited life-time of bare concrete immobilized bacteria is most likely a combination of high concrete matrix alkalinity (pH>12) and ongoing reduction in matrix pore-size diameter (<1 μm) during continued cement hydration.